Combustors, such as those used in gas turbines, for example, mix compressed air with fuel and expel high temperature, high pressure gas downstream. The energy stored in the gas is then converted to work as the high temperature, high pressure gas expands in a turbine, for example, thereby turning a shaft to drive attached devices, such as an electric generator to generate electricity. The shaft has a plurality of turbine blades shaped such that the expanding hot gas creates a pressure imbalance as it travels from the leading edge to the trailing edge, thereby turning the turbine blades to rotate the shaft.
FIG. 1 shows a gas turbine 20. Air to be supplied to the combustor 10 is received through air intake section 30 of the gas turbine 20 and is compressed in compression section 40. The compressed air is then supplied to headend 50 through air path 60. The air is mixed with fuel and combusted at the tip of nozzles 70 and the resulting high temperature, high pressure gas is supplied downstream. In the exemplary embodiment shown in FIG. 1, the resulting gas is supplied to turbine section 80 where the energy of the gas is converted to work by turning shaft 90 connected to turbine blades 95.
As shown in FIG. 2, in order to cool the turbine blades 95 where prolonged exposure to high heat can cause deformation and even structural failure, cooling holes 100 are formed on the surface of the turbine blade 95. As cooling fluid, such as cooled air, is forced out through the cooling holes 100 at high velocities, a boundary layer of cooling fluid covers the surface of the turbine blade 95 thereby cooling the turbine blade 95.
A thin steady film of cold air formed on the blade is ideal to keep the blade cool. However, typical round film holes experiences a significant reduction in film effectiveness for high blowing ratios. As shown in FIG. 3, at low (Low M) to moderate (Mod M) blowing ratios, a relatively steady boundary layer is formed from the cooling fluid escaping through the cooling hole 100 to create a cooling film 300. However, at high blowing ratios (High M), the boundary layer is disrupted by turbulence 310 and the cooling effect from the cooling fluid is significantly reduced.
In addition, the typical method of forming and ceramic coating of the film holes leaves a jagged edge around the film holes that disrupt the formation of the boundary layer thereby reducing the cooling effect. Typically, the film holes are drilled into the surface of the turbine blade using electrical discharge machining (EDM) or some form of laser. The turbine blade 95 is then coated with a thermal barrier coating (TBC) material, such as ceramic. Assuming the more common EDM manufacturing process is used and because TBC material is an insulator and EDM is only effective on metal surfaces, the film holes are formed before the coating process. Accordingly, the coating process requires plugging the film holes prior to coating the surface of the turbine blade and removing the plugging materials after the coating process is complete. The plugging material, which is typically a type of polymer, leaves a residue that creates a jagged edge around the film holes thereby reducing performance of the cooling effect.